Golf ball

ABSTRACT

A golf ball has a plurality of dimples on a surface thereof. Each dimple has a center portion and an edge portion. The center portion has a bowl shape. The edge portion has a ring shape and is smoothly continuous with the center portion. A cross-sectional shape of the edge portion is a curved line that is convex outward in a radial direction of the golf ball. A sum Vc of volumes of the center portions and a sum Vd of volumes of the dimples satisfy the following mathematical formula: (Vd−Vc)/Vc≤2.15. A ratio of a number of the dimples, the center portion of each dimple having a circular contour and the edge portion of each dimple having a non-circular contour, to a total number of the dimples, is not less than 50%.

This application claims priority on Patent Application No. 2018-162660 filed in JAPAN on Aug. 31, 2018. The entire contents of this Japanese Patent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to golf balls. Specifically, the present invention relates to dimple patterns of golf balls.

Description of the Related Art

The face of a golf club has a loft angle. When a golf ball is hit with the golf club, backspin due to the loft angle occurs in the golf ball. The golf ball flies with the backspin.

Golf balls have a large number of dimples on the surfaces thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. This phenomenon is referred to as “turbulization”. Due to turbulization, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulization promotes the displacement between the separation point on the upper side and the separation point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. The reduction of drag and the enhancement of lift force are referred to as a “dimple effect”. Excellent dimples efficiently disturb the air flow. Excellent dimples produce a long flight distance.

JP2004-73524 discloses a golf ball having dimples each including an edge and an outer portion positioned outside the edge. The outer portion contributes to the flight performance of the golf ball.

JP2006-181133 discloses a golf ball having two-tiered dimples. The two-tiered dimples contribute to the flight performance of the golf ball.

Golf players desire further improvement in flight distance. In light of flight performance, there is room for improvement of dimples. An object of the present invention is to provide a golf ball having excellent flight performance.

SUMMARY OF THE INVENTION

A golf ball according to the present invention has a plurality of dimples on a surface thereof. Each dimple has a center portion having a bowl shape, and an edge portion smoothly continuous with the center portion and having a ring shape. A cross-sectional shape of the edge portion is a curved line that is convex outward in a radial direction of the golf ball. A sum Vc of volumes of the center portions and a sum Vd of volumes of the dimples satisfy the following mathematical formula.

(Vd−Vc)/Vc≤2.15

When the golf ball according to the present invention flies, the lift force coefficient and the drag coefficient are appropriate. The golf ball has excellent flight performance.

Preferably, a ratio P of a number N1 of the dimples in each of which the center portion has a circular contour and the edge portion has a non-circular contour, to a total number N of the dimples, is not less than 50%.

Preferably, a cross-sectional shape of the edge portion is a circular arc. Preferably, a ratio (PL/CR) of a land ratio PL (%) of the surface of the golf ball to a curvature radius CR (mm) of the circular arc of the edge portion is not less than 1.0 and not greater than 5.0.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a golf ball according to an embodiment of the present invention;

FIG. 2 is an enlarged front view of the golf ball in FIG. 1;

FIG. 3 is an enlarged cross-sectional view of a part of the golf ball in FIG. 1;

FIG. 4 is an enlarged plan view of a dimple of the golf ball in FIG. 1;

FIG. 5(a) is an enlarged view of a part of the surface of the golf ball in FIG. 1;

FIG. 5(b) is an enlarged view of one dimple of the golf ball in FIG. 5(a);

FIG. 6 is a front view of a golf ball according to Comparative Example 1;

FIG. 7 is a front view of a golf ball according to Example 2 of the present invention;

FIG. 8 is a front view of a golf ball according to Example 3 of the present invention;

FIG. 9 is a front view of a golf ball according to Example 4 of the present invention;

FIG. 10 is a front view of a golf ball according to Example 5 of the present invention;

FIG. 11 is a front view of a golf ball according to Example 6 of the present invention;

FIG. 12 is a front view of a golf ball according to Example 7 of the present invention;

FIG. 13 is a front view of a golf ball according to Example 8 of the present invention;

FIG. 14 is a front view of a golf ball according to Example 9 of the present invention;

FIG. 15 is a front view of a golf ball according to Example 10 of the present invention;

FIG. 16 is a front view of a golf ball according to Comparative Example 2;

FIG. 17 is a front view of a golf ball according to Example 11 of the present invention;

FIG. 18 is a front view of a golf ball according to Example 12 of the present invention; and

FIG. 19 is a front view of a golf ball according to Example 13 of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention based on preferred embodiments with appropriate reference to the drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4, a mid layer 6 positioned outside the core 4, and a cover 8 positioned outside the mid layer 6. The golf ball 2 has a large number of dimples 10 on the surface thereof. Of the surface of the golf ball 2, a part other than the dimples 10 is a land 12. The golf ball 2 includes a paint layer and a mark layer on the external side of the cover 8, but these layers are not shown in the drawing.

The golf ball 2 preferably has a diameter of not less than 40 mm and not greater than 45 mm. From the viewpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is particularly preferably not less than 42.67 mm. In light of suppression of air resistance, the diameter is more preferably not greater than 44 mm and particularly preferably not greater than 42.80 mm.

The golf ball 2 preferably has a weight of not less than 40 g and not greater than 50 g. In light of attainment of great inertia, the weight is more preferably not less than 44 g and particularly preferably not less than 45.00 g. From the viewpoint of conformity to the rules established by the USGA, the weight is particularly preferably not greater than 45.93 g.

The core 4 is formed by crosslinking a rubber composition. Examples of the base rubber of the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. Two or more rubbers may be used in combination. In light of resilience performance, polybutadienes are preferable, and high-cis polybutadienes are particularly preferable.

The core 4 may be formed from a resin composition. The core 4 may be formed from a mixture of a rubber composition and a resin composition. A resin composition that will be described later for the mid layer 6 or the cover 8 can be used for the core 4.

The rubber composition of the core 4 includes a co-crosslinking agent. Examples of preferable co-crosslinking agents in light of resilience performance include zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate. The rubber composition preferably includes an organic peroxide together with a co-crosslinking agent. Examples of preferable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylderoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide.

The rubber composition of the core 4 may include additives such as a filler, sulfur, a vulcanization accelerator, a sulfur compound, an anti-aging agent, a coloring agent, a plasticizer, and a dispersant. The rubber composition may include a carboxylic acid or a carboxylate. The rubber composition may include synthetic resin powder or crosslinked rubber powder.

The core 4 has a diameter of preferably not less than 30.0 mm and particularly preferably not less than 38.0 mm. The diameter of the core 4 is preferably not greater than 42.0 mm and particularly preferably not greater than 41.5 mm. The core 4 may have two or more layers. The core 4 may have a rib on the surface thereof. The core 4 may be hollow.

The mid layer 6 is formed from a resin composition. A preferable base polymer of the resin composition is an ionomer resin. Examples of preferable ionomer resins include binary copolymers formed with an a-olefin and an a,-unsaturated carboxylic acid having 3 to 8 carbon atoms. Examples of other preferable ionomer resins include ternary copolymers formed with: an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. For the binary copolymer and the ternary copolymer, preferable α-olefins are ethylene and propylene, while preferable α,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. In the binary copolymer and the ternary copolymer, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for use in neutralization include sodium ion, potassium ion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymium ion.

Instead of an ionomer resin, the resin composition of the mid layer 6 may include another polymer. Examples of the other polymer include polystyrenes, polyamides, polyesters, polyolefins, and polyurethanes. The resin composition may include two or more polymers.

The resin composition of the mid layer 6 may include a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like. For the purpose of adjusting specific gravity, the resin composition may include powder of a metal with a high specific gravity such as tungsten, molybdenum, and the like.

The mid layer 6 has a thickness of preferably not less than 0.2 mm and particularly preferably not less than 0.3 mm. The thickness of the mid layer 6 is preferably not greater than 2.5 mm and particularly preferably not greater than 2.2 mm. The mid layer 6 has a specific gravity of preferably not less than 0.90 and particularly preferably not less than 0.95. The specific gravity of the mid layer 6 is preferably not greater than 1.10 and particularly preferably not greater than 1.05. The mid layer 6 may have two or more layers.

The cover 8 is formed from a resin composition. A preferable base polymer of the resin composition is a polyurethane. The resin composition may include a thermoplastic polyurethane or may include a thermosetting polyurethane. In light of productivity, the thermoplastic polyurethane is preferable. The thermoplastic polyurethane includes a polyurethane component as a hard segment, and a polyester component or a polyether component as a soft segment.

The polyurethane has a urethane bond within the molecule. The urethane bond can be formed by reacting a polyol with a polyisocyanate.

The polyol, which is a material for the urethane bond, has a plurality of hydroxyl groups. Low-molecular-weight polyols and high-molecular-weight polyols can be used.

Examples of an isocyanate for the polyurethane component include alicyclic diisocyanates, aromatic diisocyanates, and aliphatic diisocyanates. Alicyclic diisocyanates are particularly preferable. Since an alicyclic diisocyanate does not have any double bond in the main chain, the alicyclic diisocyanate suppresses yellowing of the cover 8. Examples of alicyclic diisocyanates include 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), 1,3-bis(isocyanatomethyl)cyclohexane (H₆XDI), isophorone diisocyanate (IPDI), and trans-1,4-cyclohexane diisocyanate (CHDI). In light of versatility and processability, H₁₂MDI is preferable.

Instead of a polyurethane, the resin composition of the cover 8 may include another polymer. Examples of the other polymer include ionomer resins, polystyrenes, polyamides, polyesters, and polyolefins. The resin composition may include two or more polymers.

The resin composition of the cover 8 may include a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like.

The cover 8 has a thickness of preferably not less than 0.2 mm and particularly preferably not less than 0.3 mm. The thickness of the cover 8 is preferably not greater than 2.5 mm and particularly preferably not greater than 2.2 mm. The cover 8 has a specific gravity of preferably not less than 0.90 and particularly preferably not less than 0.95. The specific gravity of the cover 8 is preferably not greater than 1.10 and particularly preferably not greater than 1.05. The cover 8 may have two or more layers.

The golf ball 2 may include a reinforcing layer between the mid layer 6 and the cover 8. The reinforcing layer firmly adheres to the mid layer 6 and also to the cover 8. The reinforcing layer suppresses separation of the cover 8 from the mid layer 6. The reinforcing layer is formed from a polymer composition. Examples of the base polymer of the reinforcing layer include two-component curing type epoxy resins and two-component curing type urethane resins.

As shown in FIG. 2, the golf ball 2 has dimples A to E. The dimples A are larger than the dimples B. The dimples B are larger than the dimples C. The dimples C are larger than the dimples D. The dimples D are larger than the dimples E. The number of the dimples A is 60; the number of the dimples B is 158; the number of the dimples C is 72; the number of the dimples D is 36; and the number of the dimples E is 12. The total number of the dimples 10 is 338. A dimple pattern is formed by these dimples 10 and the land 12.

FIG. 3 shows a cross section of the golf ball 2 along a plane passing through the central point of a dimple 10 and the central point of the golf ball 2. In FIG. 3, the top-to-bottom direction is the depth direction of the dimple 10. In FIG. 3, an alternate long and two short dashes line 14 represents a phantom sphere. The surface of the phantom sphere 14 is the surface of the golf ball 2 when it is postulated that no dimple 10 exists. The diameter of the phantom sphere 14 is equal to the diameter of the golf ball 2. The dimple 10 is recessed from the surface of the phantom sphere 14. The land 12 coincides with the surface of the phantom sphere 14. In FIG. 3, reference character EP represents an edge point of the dimple 10, and reference character JP represents a joint point (described in detail later).

FIG. 4 is an enlarged plan view of a dimple 10 of the golf ball 2 in FIG. 1. In FIG. 4, reference character EL represents an edge line of the dimple 10. The edge line EL is a set of a large number of edge points EP (see FIG. 3). Of the surface of the golf ball 2, a part surrounded by the edge line EL is the dimple 10. Of the surface of the golf ball 2, the outside of the edge line EL is the land 12 (see FIG. 2).

The dimple 10 has a center portion 16 and an edge portion 18. In FIG. 4, reference character JL represents a joint line. The joint line JL is a set of a large number of joint points JP (see FIG. 3). Of the dimple 10, a part surrounded by the joint line JL is the center portion 16. Of the dimple 10, a part between the edge line EL and the joint line JL is the edge portion 18.

As shown in FIGS. 3 and 4, the center portion 16 has a bowl shape. A cross-sectional shape of the center portion 16 is a curved line that is convex inward in the radial direction of the golf ball 2. In the present embodiment, the cross-sectional shape of the center portion 16 is a circular arc. The cross-sectional shape of the center portion 16 may be a combination of a plurality of circular arcs. The cross-sectional shape of the center portion 16 may be a curved line other than a circular arc. The cross-sectional shape of the center portion 16 may be partially convex outward. In the present embodiment, the contour of the center portion 16 (that is, the joint line JL) is a circle.

As shown in FIGS. 3 and 4, the edge portion 18 has a ring shape. A cross-sectional shape of the edge portion 18 is a curved line that is convex outward in the radial direction of the golf ball 2. In the present embodiment, the cross-sectional shape of the edge portion 18 is a circular arc. The cross-sectional shape of the edge portion 18 may be a combination of a plurality of circular arcs. The cross-sectional shape of the edge portion 18 may be a curved line other than a circular arc.

In the present embodiment, the curved line of the edge portion 18 is tangent to a curved line of the phantom sphere 14. In other words, the edge portion 18 is smoothly continuous with the phantom sphere 14. Therefore, in the actual golf ball 2, the edge line EL is not clearly viewed as an edge.

The curved line of the cross-section of the center portion 16 and the curved line of the cross-section of the edge portion 18 are tangent to each other at the joint point JP. In other words, the edge portion 18 is smoothly continuous with the center portion 16. Therefore, in the actual golf ball 2, the joint line JL is not clearly viewed as an edge.

According to the finding by the present inventor, the dimple 10 in which the center portion 16 has a bowl shape and the cross-sectional shape of the edge portion 18 is a curved line that is convex outward, contributes to a low drag coefficient. The golf ball 2 has excellent flight performance.

The effect by the dimple 10 is achieved particularly upon a shot with a middle iron. When the golf ball 2 is hit with a middle iron, a low drag coefficient is obtained in a trajectory after the peak.

In the present invention, a ratio Rv is calculated by the following mathematical formula.

Rv=(Vd−Vc)/Vc

In this mathematical formula, Vc represents the sum of the volumes of all the center portions 16, and Vd represents the sum of the volumes of the dimples 10. The volume of each center portion 16 is the volume of a space surrounded by the surface of the dimple 10 and a plane including the joint line JL. The volume of each dimple 10 is the volume of a space surrounded by the surface of the dimple 10 and the surface of the phantom sphere 14. The ratio Rv is preferably not greater than 2.15. In other words, the golf ball 2 preferably satisfies the following mathematical formula.

(Vd−Vc)/Vc≤2.15

In the golf ball 2 in which the ratio Rv is not greater than 2.15, the center portions 16 sufficiently contribute to flight performance. From this viewpoint, the ratio Rv is more preferably not greater than 2.10 and particularly preferably not greater than 2.05. The ratio Rv is preferably not less than 1.00. In the golf ball 2 in which the ratio Rv is not less than 1.00, the edge portions 18 sufficiently contribute to flight performance. From this viewpoint, the ratio Rv is more preferably not less than 1.05 and particularly preferably not less than 1.10.

A land ratio PL of the surface of the golf ball 2 is preferably not greater than 12%. The land ratio PL is the ratio of the area of the land 12 to the surface area of the phantom sphere 14. The golf ball 2 in which the land ratio PL is not greater than 12% has excellent flight performance. From this viewpoint, the land ratio PL is more preferably not greater than 10% and particularly preferably not greater than 8%.

In FIG. 3, an arrow CR represents the curvature radius of the circular arc of the edge portion 18. The curvature radius CR is preferably not less than 1.25 mm. The edge portion 18 having a curvature radius CR of not less than 1.25 mm contributes to a low drag coefficient. From this viewpoint, the curvature radius CR is preferably not less than 1.50 mm and particularly preferably not less than 1.75 mm. From the viewpoint that the center portion 16 can have a sufficient volume, the curvature radius CR is preferably not greater than 4.0 mm.

In each dimple 10, the curvature radius CR is preferably within the above range. In only some of the dimples 10, the curvature radius CR may be within the above range. The ratio of the number of the dimples 10 each having a curvature radius CR within the above range, to the total number N of the dimples 10, is preferably not less than 50%, more preferably not less than 70%, and particularly preferably not less than 90%.

The ratio (PL/CR) of the land ratio PL (%) of the surface to the curvature radius CR (mm) is preferably not less than 1.0 and not greater than 5.0. With the golf ball 2 in which the ratio (PL/CR) is within this range, a low drag coefficient can be achieved. From this viewpoint, the ratio (PL/CR) is more preferably not less than 1.5 and particularly preferably not less than 2.0. The ratio (PL/CR) is more preferably not greater than 4.7 and particularly preferably not greater than 4.4.

In each dimple 10, the ratio (PL/CR) is preferably within the above range. In only some of the dimples 10, the ratio (PL/CR) may be within the above range. The ratio of the number of the dimples 10 in each of which the ratio (PL/CR) is within the above range, to the total number N of the dimples 10, is preferably not less than 50%, more preferably not less than 70%, and particularly preferably not less than 90%.

The sum Vd of the volumes of the dimples 10 is preferably not less than 530 mm³ and not greater than 630 mm³. With the golf ball 2 in which the sum Vd is not less than 530 mm³, rising of the golf ball 2 during flight is suppressed. From this viewpoint, the sum Vd is more preferably not less than 550 mm³ and particularly preferably not less than 560 mm³. With the golf ball 2 in which the sum Vd is not greater than 630 mm³, dropping of the golf ball 2 during flight is suppressed. From this viewpoint, the sum Vd is more preferably not greater than 610 mm³ and particularly preferably not greater than 600 mm³.

FIG. 5(a) is an enlarged view of a part of the surface of the golf ball 2 in FIG. 1. FIG. 5(a) shows one dimple 10 a and six dimples 10 b surrounding the dimple 10 a. An edge portion 18 a of the dimple 10 a is adjacent to each of edge portions 18 b of the other dimples 10 b such that the edge portion 18 a shares a boundary line 20 with the edge portion 18 b.

FIG. 5(b) is an enlarged view of the dimple 10 a of the golf ball 2 in FIG. 5(a). The contour of a center portion 16 a of the dimple 10 a is circular. The contour of the edge portion 18 a of the dimple 10 a is non-circular. The contour of the edge portion 18 a includes six circular arcs 22 and six boundary lines 20. The contour of the edge portion 18 a is a shape when the center of the dimple 10 a is viewed at infinity. In the present embodiment, each boundary line 20 is a curved line that is convex outward in the radial direction of the golf ball 2. The boundary line 20 can be approximated to a straight line.

The center portion 16 having a circular contour is excellent in aerodynamic symmetry of the golf ball 2. The edge portion 18 having a non-circular contour contributes to a low land ratio PL. The golf ball 2 having the dimple 10 in which the center portion 16 has a circular contour and the edge portion 18 has a non-circular contour, has excellent aerodynamic symmetry and flight performance.

In the present invention, a ratio P is calculated by the following mathematical formula.

P=(N1/N)*100

In this mathematical formula, N1 is the number of the dimples 10 in each of which the center portion 16 has a circular contour and the edge portion 18 has a non-circular contour, and N is the total number of the dimples 10.

The ratio P is preferably not less than 50%. The golf ball 2 in which the ratio P is not less than 50% has excellent aerodynamic symmetry and flight performance. From this viewpoint, the ratio P is more preferably not less than 70% and particularly preferably not less than 90%. The ratio P is ideally 100%.

EXAMPLES Example 1

A rubber composition was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 22.5 parts by weight of zinc diacrylate, 5 parts by weight of zinc oxide, 5 parts by weight of barium sulfate, 0.5 parts by weight of diphenyl disulfide, and 0.6 parts by weight of dicumyl peroxide. This rubber composition was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 18 minutes to obtain a core with a diameter of 38.5 mm.

A resin composition was obtained by kneading 50 parts by weight of an ionomer resin (trade name “Himilan 1605”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.), 50 parts by weight of another ionomer resin (trade name “Himilan AM7329”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.), and 4 parts by weight of titanium dioxide with a twin-screw kneading extruder. The core was covered with this resin composition by injection molding to form a mid layer with a thickness of 1.6 mm.

A paint composition (trade name “POLIN 750LE”, manufactured by SHINTO PAINT CO., LTD.) including a two-component curing type epoxy resin as a base polymer was prepared. The base material liquid of this paint composition includes 30 parts by weight of a bisphenol A type solid epoxy resin and 70 parts by weight of a solvent. The curing agent liquid of this paint composition includes 40 parts by weight of a modified polyamide amine, 55 parts by weight of a solvent, and 5 parts by weight of titanium dioxide. The weight ratio of the base material liquid to the curing agent liquid is 1:1. This paint composition was applied to the surface of the mid layer with a spray gun, and kept at 23° C. for 6 hours to obtain a reinforcing layer with a thickness of 10 μμm.

A resin composition was obtained by kneading 100 parts by weight of a thermoplastic polyurethane elastomer (trade name “Elastollan XNY85A”, manufactured by BASF Japan Ltd.) and 4 parts by weight of titanium dioxide with a twin-screw kneading extruder. Half shells were obtained from this resin composition by compression molding. The sphere consisting of the core, the mid layer, and the reinforcing layer was covered with two of these half shells. These half shells and the sphere were placed into a final mold that includes upper and lower mold halves each having a hemispherical cavity and having a large number of pimples on its cavity face, and a cover was obtained by compression molding. The thickness of the cover was 0.5 mm. Dimples having a shape that is the inverted shape of the pimples were formed on the cover.

A clear paint including a two-component curing type polyurethane as a base material was applied to this cover to obtain a golf ball of Example 1 with a diameter of about 42.7 mm and a weight of about 45.6 g. The dimple pattern of the golf ball is shown in FIG. 2. The specifications of the dimples of the golf ball are shown in Table 1 below. Each dimple has a center portion and an edge portion. The curvature radius CR of the edge portion is 2.0 mm. In the dimple pattern, a ratio P1 of the number of the dimples in each of which the center portion has a circular contour, to the total number N of the dimples, is 100%. A ratio P2 of the number of the dimples in each of which the edge portion has a non-circular contour, to the total number N of the dimples, is 100%. The ratio P of the number N1 of the dimples in each of which the center portion has a circular contour and the edge portion has a non-circular contour, to the total number N of the dimples, is 100%. The land ratio PL is 7.7%. The sum Vd of the volumes of the dimples is 569 mm³.

Examples 2 to 13 and Comparative Example 1

Golf balls of Examples 2 to 13 and Comparative Example 1 were obtained in the same manner as Example 1, except the final mold was changed and the specifications of the dimples were set as shown in Tables 1 to 4 below.

Comparative Example 2

A golf ball of Comparative Example 2 was obtained in the same manner as Example 1, except the final mold was changed and the specifications of the dimples were set as shown in Table 4 below. Each dimple of the golf ball has an almost bowl shape.

[Flight Test]

An iron type golf club #7 (trade name “XXIO-10”, manufactured by Sumitomo Rubber Industries, Ltd., shaft hardness: R) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under the condition of a head speed of 33 m/sec, and the distance from the launch point to the stop point was measured. During the test, the weather was almost windless. The average value of data obtained by 20 measurements is shown in Tables 1 to 4 below.

TABLE 1 Results of Evaluation Comp. Example 1 Example 1 Example 2 Front view FIG. 6 FIG. 2 FIG. 7 Vc (mm³) 186 210 235 Vd − Vc (mm³) 403 359 313 (Vd − Vc)/Vc 2.17 1.70 1.33 P1 (%) 100 100 100 P2 (%) 100 100 100 P (%) 100 100 100 CR (mm) 2.5 2.0 1.5 PL (%) 5.6 7.7 12.1 PL/CR 2.2 3.9 8.1 Flight 147.1 149.9 149.4 distance (m)

TABLE 2 Results of Evaluation Example 3 Example 4 Example 5 Example 6 Front view FIG. 8 FIG. 9 FIG. 10 FIG. 11 Vc (mm³) 194 218 241 265 Vd − Vc (mm³) 409 375 336 297 (Vd − Vc)/Vc 2.11 1.72 1.39 1.12 P1 (%) 100 100 100 100 P2 (%) 100 100 100 100 P (%) 100 100 100 100 CR (mm) 2.5 2.0 1.5 1.0 PL (%) 3.8 5.8 9.6 12.5 PL/CR 1.5 2.9 6.4 12.5 Flight 148.5 150.2 149.5 148.7 distance (m)

TABLE 3 Results of Evaluation Example 7 Example 8 Example 9 Example 10 Front view FIG. 12 FIG. 13 FIG. 14 FIG. 15 Vc (mm³) 203 225 247 270 Vd − Vc (mm³) 428 395 362 329 (Vd − Vc)/Vc 2.11 1.76 1.46 1.22 P1 (%) 100 100 100 100 P2 (%) 100 100 100 100 P (%) 100 100 100 100 CR (mm) 2.5 2.0 1.5 1.0 PL (%) 1.9 3.9 7.0 9.0 PL/CR 0.8 2.0 4.7 9.0 Flight 148.6 149.8 149.8 149.1 distance (m)

TABLE 4 Results of Evaluation Comp. Example Example Example Example 2 11 12 13 Front view FIG. 16 FIG. 17 FIG. 18 FIG. 19 Vc (mm³) — 227 230 235 Vd − Vc (mm³) — 292 297 317 (Vd − Vc)/Vc — 1.29 1.29 1.35 P1 (%) — 100 100 100 P2 (%) — 47 57 82 P (%) — 47 57 82 CR (mm) — 1.5 1.5 1.5 PL (%) 18 15.1 14.3 12.6 PL/CR — 10.8 10.1 8.4 Flight 148.0 148.5 148.8 149.3 distance (m)

As shown in Tables 1 to 4, the golf ball of each Example has excellent flight performance. From the evaluation results, advantages of the present invention are clear.

The aforementioned dimple pattern is applicable to golf balls having various structures such as a one-piece golf ball, a two-piece golf ball, a four-piece golf ball, a five-piece golf ball, a six-piece golf ball, a thread-wound golf ball, and the like in addition to a three-piece golf ball. The above descriptions are merely illustrative examples, and various modifications can be made without departing from the principles of the present invention. 

1. A golf ball having a plurality of dimples on a surface thereof, wherein each dimple has a center portion having a bowl shape, and an edge portion smoothly continuous with the center portion and having a ring shape, wherein a cross-sectional shape of the edge portion is a curved line that is convex outward in a radial direction of the golf ball, and wherein a sum Vc of volumes of the center portions and a sum Vd of volumes of the dimples satisfy the following mathematical formula: (Vd−Vc)/Vc≤2.15.
 2. The golf ball according to claim 1, wherein a ratio of a number of the dimples, the center portion of each dimple having a circular contour and the edge portion of each dimple having a non-circular contour, to a total number of the dimples, is not less than 50%.
 3. The golf ball according to claim 1, wherein the cross-sectional shape of the edge portion is a circular arc, and wherein a ratio (PL/CR) of a land ratio PL (%) of the surface of the golf ball to a curvature radius CR (mm) of the circular arc is not less than 1.0 and not greater than 5.0. 